A description of laser ablation in the publication of Arrowsmith and Hughes, APPLIED SPECTROSCOPY, 42, 7, 1988 (1231-1239) is commonly cited as the beginning of modern analytical laser ablation for inductively coupled plasma (ICP) emission and inductively coupled plasma mass spectrometry (ICP-MS) analysis of solid samples. More recently, the July 2008 issue of Gases & Instruments features an article by Hughes, Brady, and Fry which reviews the use of UV lasers and analytical laser ablation in general, with illustration of how white light illumination and viewing is normally done for UV laser ablation and discussion of parameters affecting ablation quality, ablation morphology, ablation rate, and related aerosol particle size from an ablation event. From the G&I article it should be noted that an opto-mechanical (OM) ablation is desired for analytical laser ablation, rather than a thermal process. It should also be noted from the G&I article that a small particle size is desired in the aerosol resulting from an ablation event, to ensure efficient aerosol mass transport (to the external analytical instrument) and to minimize overall system calibration difficulty and variability.
FIGS. 1A, 1B illustrate that a dichroic mirror (6) is normally used in prior art analytical UV laser ablation, to allow a view camera (22) to view a solid material (11, 24) coaxially (18, 19, 20) with a final segment of the UV laser beam (7, 10, 24) which is also focused to ablate the surface of the solid material (11, 24). The prior art dichroic mirror (6) has a very thin film mirror coating which is highly reflective only to light of a specific UV laser wavelength, which is the specific “design” wavelength of the particular (dichroic) laser mirror in question, and which is based on selective constructive interference (in the reflection mode) of light at that design wavelength, exclusively. All other wavelengths (shorter and longer than the specific UV design wavelength) are not reflected. Instead, the thin mirror coating is transparent to the other wavelengths (e.g. visible light) and passes them like a window (even if angled). FIGS. 1A, 1B thereby illustrate that the prior art UV laser beam (5, 7) may be efficiently reflected from the angled side of an appropriately designed UV dichroic mirror (6) towards a solid sample surface (11, 24), while an overhead visible “white light” camera (20, 22) view may be taken through the same prior art angled UV dichroic laser mirror (6) from the top, since the UV dichroic laser mirror (6) is transparent to visible (e.g. “white”) light.
The prior art objective lens (8) performs two functions. First it focuses the UV laser beam (7, 10) downward onto the solid sample material surface (24); second, it simultaneously operates (in reverse) to coaxially focus a visible, white light image of the solid sample surface (24) upward (18, 19, 20) to the camera (22) focal plane. (It should be noted that an auxiliary visible, white light source, e.g. ring illuminator (16) is typically also provided to coaxially illuminate (17) a relatively wide area (e.g. 1-10 mm) of the solid sample surface (11, 24) continuously (to light the “subject” for the camera view), while the pulsed, Q-switched UV laser fires (flashes) intermittently (10), but repetitively to ablate a smaller spot (24, e.g. 0.02-0.2 mm) on the solid sample surface.)
The disadvantage of the prior art coaxial camera view of laser ablation in FIGS. 1A, 1B is that both the laser beam (7, 10) and the camera view (18, 19, 20, 22) must pass through the same short focal length prior art objective lens (8), albeit in opposite directions, so the two prior art optical paths are (undesirably) coupled. The prior art objective lens (8) must be designed to efficiently pass (7, 10) UV laser radiation (e.g. 193 nm, 213 nm, and 266 nm) with high transparency at UV wavelengths. The available optical materials (for doing that) do not simultaneously allow an ideally achromatic focused visible (e.g. 400 nm-700 nm) “white light” view for the prior art camera path (18, 19, 20, 22). The prior art UV laser objective lens (8) can produce a good quality monochromatic UV laser image (24), but then it is not achromatic for longer wavelength visible light and therefore cannot focus both ends of the white light spectrum (red and blue) simultaneously in the same prior art camera plane (22). Undesirable chromatic aberration thus arises for the prior art “white light” on-axis view, in order to ensure a good UV laser ablation experiment. A poorly focused prior art “white light” (camera) image (22) therefore adversely affects many analytical UV laser ablation systems today, and there remains a need to decouple the visible white light camera view (22) from the chromatic aberration of a UV laser objective lens (8).
A second disadvantage of prior art UV analytical laser ablation is that low ablation rates and poor sensitivity for bulk solid analysis typically result under conditions where a high quality opto-mechanical (hereafter OM) ablation occurs with prior art UV analytical laser ablation systems. The basic problem arises from a situation where UV analytical laser ablation (for ICP and ICP-MS) is a relatively new field, with complete (integrated) prior art analytical systems becoming commercially available for the first time in 1995. With UV analytical laser ablation still in its “infancy” (a relatively small number of installations as of this writing), prior art commercial analytical laser ablation manufacturers are both small in size and few in number. Thus far (1995-present), the small group of prior art analytical laser ablation manufacturers have primarily been designing prior art products tailored to the needs of a narrowly focused group of customers working in Geology.
Geologists have certainly done the infant analytical laser ablation field a significant service by purchasing prior art commercial units early in its manufacturing development cycle, thereby making the infant field of analytical laser ablation commercially viable (albeit on a relatively small commercial scale). Through effective lobbying, they (geologists) have influenced the small group of prior art analytical laser ablation manufacturers and successfully imposed their own particular (geologic) biases onto the features and characteristics of commercially available prior art system configurations. The few existing prior art analytical laser ablation manufacturers have therefore catered primarily to the (prior art) geologic “configuration bias” (hereafter, “geo-bias”), rather than designing flexible, general purpose analytical laser ablation systems of the type that would be needed for widespread usage for bulk analysis of solid materials in general, for a wider variety of laboratories.
The prior art geo-bias typically dictates a small, homogeneous focused spot diameter, since geologists are typically interested in elemental analysis of small inclusions and other small heterogeneities in rocks and minerals. Consequently, focused laser spot diameters as small as 2 micron are desired in the geo-bias, and prior art excimer and SMR analytical UV laser ablation systems are not sold with a homogeneous focused spot diameter larger than 200 microns. Prior art SMR analytical UV laser ablation objective lenses (8, FIG. 1A-1B) to produce such small homogeneous spot diameters typically exhibit short focal length (e.g. F=18-38 mm) and their working distance (to the sample surface) is only slightly more than that. This prior art geo-bias for short focal length objective lenses and small spot diameters is ideal for geologists interested in analyzing small isolated features in heterogeneous rocks and minerals, but it is not ideal for high sensitivity bulk solids analysis or more homogeneous sample materials in other fields.
The short focal length objective lens and small spot diameters characteristic of the geo-bias in prior art SMR analytical laser ablation, actually preclude using high laser power to enhance sensitivity. In fact, there is a certain maximum laser power that can be optimally employed for prior art focused laser spots of 200 microns diameter and less, which is the largest homogeneous focused spot available in commercial prior art excimer and SMR analytical laser ablation systems. In prior art analytical laser ablation manufacturing, the geo-bias therefore leads to use of relatively small excimer and SMR UV lasers (less than 12 mJ at 266 nm) and short laser path lengths. This keeps the system size and price down, but it also limits the sensitivity which can be obtained in bulk solids analysis with a prior art system.
At ETH-Zurich, Guenther, Horn, and Guillong employed larger prior art Gaussian beam (TEM 00) lasers with external prior art beam homogenizing optics, and a large excimer laser was substituted in a commercial prior art system (Geo-Lase by Coherent, distributed for several years by CETAC), but in both cases, the external beam homogenizers were inefficient (subject to significant light transmission loss), the firing frequency reduced to 10 Hz maximum for the TEM 00 Nd-YAG laser, and the objective lenses were characterized by the short geo-bias focal length (F<40 mm) and relatively small maximum focused spot diameter in both cases, so the actual final output (relating to ablation rate) of these prior art systems was only slightly more than the smaller, more efficient, prior art frequency-multiplied SMR Nd-YAG analytical UV laser ablation systems, and sensitivity for bulk analysis wasn't appreciably enhanced with either of these two larger prior art lasers and their associated ablation systems.
One larger (40 mJ) prior art commercial Nd-YAG laser ablation system operating at 266 nm, 10 Hz was coupled to a maximum focused spot size of 780 μm (0.78 mm), but this prior art laser ablation system (MACRO by New Wave, Inc.) wasn't designed for operation in the SMR mode to produce a homogenized beam profile. It was instead an unstable multimode resonator (UMR) with a gradient reflectance mirror (GRM), by design. The prior art GRM unstable resonator is actually designed for small spot focusing (low divergence rate, compared to SMR) and it is well known that the GRM unstable multimode resonator (UMR) does not produce the desirable homogenized beam profile for large spots, and is instead characterized by a “donut with hole” or “scooped” beam profile. Initial laser ablation testing with this prior art GRM unstable resonator (UMR) analytical laser ablation system determined that it was not a reliable configuration at high power (40 mJ, 266 nm). This prior art unstable resonator (UMR) deteriorated rapidly in terms of power output and ablation crater quality. In summary, the GRM unstable multimode resonator (UMR) beam profile is not homogeneous like an SMR, and the limited prior art firing frequency of 10 Hz further reduces the sensitivity of a MACRO system relative to 20 Hz SMR system. Finally, the energy output of this unstable resonator has been reported to be erratic and frequently dropping to 20 mJ instead of the 40 mJ UMR rating.
There remains a need for high sensitivity analytical UV laser ablation based on a stable, reliable, high powered (e.g >12 mJ @ 266 nm, with similar higher powered 213 nm and 193 nm systems) homogenous beam SMR (stable multimode resonator) laser with a firing frequency higher than 10 Hz and a laser objective lens with focal length greater than F=40 mm with reduced demagnification to produce larger homogeneous focused spot diameters (>200 um) commensurate with higher laser power to achieve high analytical sensitivity within the ideal irradiance range (IIR) of solid sample materials.
The referenced G&I article indicated that each different solid sample material has a relatively narrow range of focused laser irradiance (joules/cm2/ns) which is ideal for producing the best OM ablation characteristics. Operating within the ideal irradiance range (IIR) for a given material minimizes thermal ablation effects (which otherwise make calibration more difficult and unreliable) and yields the smallest aerosol particle size. If the focused laser ablation irradiance is lower than the IIR for a given material, then thermal ablation predominates, ablation rates are low, calibration is difficult and unreliable, and analytical sensitivity is poor. If the focused laser irradiance is higher that the IIR of a material, then that sample is “over powered” and undesirable sample shattering and cracking occurs, destabilizing the analytical instrument response without significantly improving the sensitivity. In this case, ablation is too violent (rough) and too many large particles are blown out of the ablation crater, the large particles being too large for efficient transport to the external instrument. They wind up splattered throughout the ablation cell, settling out on various cell and tubing wall surfaces without transporting to the plasma or contributing appreciably to the analysis. In such an overpowered situation, the signal in the external instrument becomes temporally unstable. The ideal irradiance range (IRR) should therefore be maintained for each material and should not be exceeded.
For small focused spot diameters (<200 urn) characteristic of the geo-bias, the IIR is matched with relatively small, low powered SMR UV lasers and relatively short prior art laser paths and short focal length prior art objective lenses. For example, for a 266 nm (4th harmonic) pulsed, Q-switched, Nd-YAG laser, SMR systems in the range of 9-12 mJ are about the limit of useful laser size, in prior art commercial systems where the geo-bias prevails to limit the maximum focused prior art spot diameter to 200 microns or less. Larger lasers of 30 mJ, 40 mJ, 50 mJ, 60 mJ, 90 mJ, and 230 mJ are available at 266 nm and the desired SMR mode, but these have typically not been used for prior art analytical laser ablation, simply because the geo-bias prevailing in that industry precludes their usage in prior art short path applications with a focused spot size range 2-200 microns, where they would simply over-power the ideal irradiance range (IIR) of virtually all solid samples.
The overall result of favoring smaller lasers, shorter path lengths, and limited spot diameter (geo-bias in the prior art analytical laser ablation industry) is that prior art system sizes and prices are “contained”, but analytical sensitivities in this prior art configuration are limited to the part-per-million (ppm) range for ICP and ICP-MS analysis of solids. There is no reported prior art high sensitivity (part-per-billion, ppb) analytical UV laser ablation system based on a stable multi-mode resonator (SRM) and which produces homogeneous focused spot diameters up to 1.5 mm (in a preferred embodiment) and allows use of pulsed SMR 266 nm lasers as large as 50 mJ-230 mJ in a long path length configuration, or other equivalently oversized UV lasers at even shorter wavelength, while still operating within the optimized IIR of solid materials.
An invention is therefore needed for analytical UV laser ablation in which the ppm (part-per-million) sensitivity limitations of the prior art short laser path, short objective focal length, high demagnification ratio and small spot geo-bias would be removed via replacement with a more sensitive analytical laser ablation invention employing longer laser path lengths and longer focal length objective lenses in a ratio favoring lower demagnification ratios and larger spot diameters from larger SMR lasers operating at full power, coupling most of their energy into the sample without exceeding IIR values of solid materials to be analyzed. This would enhance the sensitivity of bulk analysis by analytical laser ablation and lead to a new era of high sensitivity (ppb (part-per-billion)) bulk analysis in the solid phase. It would be further desirable if this were achieved simultaneously with the aforementioned invention decoupling of laser focusing from white light focusing.
Since the ideal irradiance range (IIR) varies widely in solid materials, but is a relatively narrow range for each material, it is apparent that conventional systems with fixed demagnification ratio have a limited ability to maintain the IIR of each material in a wide range of solid materials, while simultaneously running the system at 100% laser power and using the full laser beam to maximize ablation rates. If a larger laser were selected, optical attenuation or power attenuation could be employed to “throttle it back” and keep all samples within their respective IIR's, but if the demagnification ratio is fixed as with prior art systems, ablation rates will not be kept at the maximum possible ablation rate for that laser over a wide range of solid sample materials.
There remains a need for an invention which would allow wide range, operationally variable demagnification ratio (operationally variable maximum spot diameter), so that the laser may be operated at 100% power output while the ablation proceeds within the IIR of each solid material by simply having the spot diameter adjusted so that 100% of the laser power is delivered within the IIR of that material. This could theoretically be done to a limited extent with a turret holding 2 to 4 different interchangeable objective lenses to yield several different demagnification ratios, but the number and range of focal lengths which may be accommodated in a single turret (for a fixed turret-to-laser head “object” distance and a limited range of turret-to-sample image distance variation) is limited to about 3 or 4 lenses whose focal lengths are not widely varying (one from the other). The IIR of solid materials varies more widely than an objective lens turret could cover by itself. In order to accommodate a wider range of IIR, an invention with an operationally variable laser “object” distance (over wide range) and an operationally variable laser “image” distance (over wide range) is also needed (or needed instead). Essentially, there remains a need for a laser ablation system with operationally variable path (over a large range of path length) to create a larger range of demagnification ratios for each objective lens. Such an invention would benefit both analytical laser ablation and laser micro-machining applications.
A third disadvantage of prior art analytical laser ablation is that the currently prevailing geo-bias involving relatively short prior art laser path lengths and short prior art laser objective focal lengths yields a shallow depth-of-focus (in the focused prior art laser spot) of only about 0.25 mm or less. If the sample surface roughness, topography, flatness, or deviation from parallel (to ablation cell horizontal translation axis when mounted in cell) varies by more than this, different locations on the sample surface must be refocused upon changing location in a prior art system. For a laser ablation line scan or raster pattern involving controlled (motorized, FIG. 1B, items 47-52) horizontal sample motion (during ablation), it is obvious that the sample flatness (and degree to which the sample surface is held parallel to the axis of motion) must be less than the depth-of-focus of the laser spot doing the ablation, otherwise the spot will lose focus and the ablation rate will change during the horizontal motion scan or raster on the sample surface. This typically means that the sample must be flat and parallel to the motion axis, within 0.25 mm (250 μm) or less in a prior art system, and it often requires that solid samples with surface roughness or uneven topography (greater than this) must be cut or ground flat, prior to ablation.
As one of the principal advantages of laser ablation (compared to acid dissolution of solid samples prior to ICP or ICP-MS analysis with nebulizer introduction of the resulting liquid) is supposed to be “elimination of sample preparation”, the oft-required cutting, grinding, or pelletizing of irregular surfaced solid samples for conventional prior art laser ablation is clearly counter-productive. There remains a need for an analytical laser ablation invention with increased depth-of-focus (in the focused laser spot) from the current prior art range of 0.25 mm (or less) to a much larger invention depth-of-focus such as 1 mm or even 2 mm, to accommodate greater surface roughness and larger variation in surface topography for laser ablation analysis without prior sample preparation or resurfacing by cutting, grinding, or pelletizing.
A fourth disadvantage to prior art laser ablation is the lack of an automated sample changer, which (lack) prevents automated sequential analysis of a large group of samples, or even a small group of samples if they are too large for more than one of them to fit into the ablation cell at any one time. Many reasons preclude the use of an auto-sampler in prior art analytical laser ablation. For one example, the short focal length prior art objective lenses (geo-bias) typically do not allow room for the sample cell to be automatically opened while positioned under the objective lens. There remains a need for development of an automatic (mechanized) sample changer for analytical laser ablation.
A fifth disadvantage of prior art analytical laser ablation is that, in the field of high activity nuclear waste analysis, prior art analytical UV laser ablation has heretofore not been well suited to a radiation “hot cell” environment, due to rapid prior art laser ablation component failure upon exposure to high level radioactivity. Typical key conventional prior art laser ablation component failures occur within 500-1,000 rads total accumulated exposure. With high activity nuclear waste samples in a hot cell, exposure rates of 1,000-2,000 rads/hour are to be expected. This means key conventional prior art laser ablation components would fail within 1 hour or less, and sometimes within 15 minutes. This is true of prior art small motors, optical coatings, electronic circuits—especially integrated circuits, laser heads, power supplies, sensors, and video cameras. Additional prior art components subject to failure on a somewhat longer time scale (still problematic) include cables, connectors, insulation on wires, o-rings, lubricants, adhesives, and a variety of plastic or polymer parts, as well as conventional optics. Laser mirrors (thin film dichroic) are particularly susceptible to radiation damage. Conventional prior art analytical UV laser ablation systems can't even withstand 1 day in the hot cell with high activity nuclear waste samples which nevertheless require analysis.
In a March 2007 government report (07-DESIGN-042, U.S. DOE Office of River Protection, contract DE-ACO5-76RL01830), the US DOE has designated laser ablation as a critical technology element (CTE) necessary for the $12.3B nuclear waste processing (vitrification) plant now under construction at the DOE Hanford, WA site. It would therefore be desirable if an invention comprehensively rad-hardened laser ablation system could be developed to withstand 1,000-2,000 rads per hour for an expected useful life of 7-12 years in that environment, instead of failing within less than a day, or less than 1 hour. A total radiation tolerance of 100 million rads total accumulated exposure is therefore desired for an invention comprehensively rad-hardened laser ablation system for nuclear waste analysis. With prior art UV laser ablation systems failing within 500-1,000 rads total accumulated exposure, it is clear that there remains a need for a new invention to meet DOE radiation hot cell needs.